I have hedman 79250 headers on my 88-440 truck

mine has all 12 stock studs, I did clean up holes while heads were on bench.

what I found was I had been putting the gasket on upside down. a machined groove across head would not let the gasket sit flat on the head around the ports. once I found that I have had no leaks at the header flange. lower collector is a ball/cone so no gaskets there.

I have the plug wire run/loomed behind the tubes with no issues. I do have heat socks on them for added protection. all 90* plug boots. same routing a 78 BB MH used with 2 center dump manifolds.

IIRC these are the same as the Bouchillon BB truck swap headers for 72-87 D series chassis.

D300 is a big open truck chassis with a solid front axle/leaf spring correct?

these headers tuck up nice and don't hang low in the chassis which is a plus sometimes.